Category Archives: Mechanical Testing

I’ve covered, welding and weld defect in past posts, what I missed completely is the testing of Weldments.

One of the purpose of effective quality control program is to determine the suitability of a given base metal or a weld to perform its intended service.

When It comes to testing, we have two type of testing.

1. Destructive/Mechanical Testing &

2. Non Destructive Testing

Destructive Testing/Mechanical Testing :

The term mechanical testing is used to describe a group of test methods for establishing or confirming the mechanical properties of a material or a completed weld. Most of these tests involve sectioning or otherwise destroying some part of the object being tested and thus they are sometimes called destructive tests. The tests are generally classified by the property they are intended to define. Each follows a well-established procedure, which is part of a published standard, allowing individual test results to be compared to other results or statistical norms. This section describes the following mechanical tests, some of which are destructive, that are carried out on welds:

o Macro Test:To see the penetration inside the parent material, and to qualify welder & welding procedure, generally a chemical etching is done on the weld specimen, see the image below.

Macro Specimen

o Bend Test :

The bend test is a popular test method that is found in many welding standards and specifications throughout the world due to the simplicity of the test method and equipment required. The history of the bend test dates back to the early years of wrought iron and steel testing before the advent of modern testing equipment. Bend specimens are prepared typically from a test plate rather than from an expensive finished product and are used to evaluate the ductility and soundness of welded joints.

There are two different bend testing methods:

Guided bend test

Free bend test

o. Guided Bend Test

The guided bend test is commonly used in welder and procedure qualification tests to determine the ability of the welder to make sound welds. The test is performed by bending prepared specimens of a specific dimension (usually specified in the relevant code) in a special jig. The dimensions of the jig will vary with specimen thickness and material.

It is important to note that the strain applied to the test specimen depends on the spacing of the rollers and the radius of the male member. The strain on the outside fibre of the bend specimen can be approximated from the following formula : e = 100 t / ( 2R + t )

When performing qualification tests the specimen thickness and bend radius are chosen according to the ductility of the metal being tested. An elongation in the outside fibre of 20 percent can be easily achieved on sound mild steel welds. Bend tests will consistently fail if the specimens contain weld discontinuities that are on are near the surface of the material.

After bending, the welds are examined for the presence of discontinuities. Many welding standards and specifications consider that a bend specimen has failed if on examination of the convex surface after bending there is a crack or open defect exceeding 3mm (1/8 in.). ﻿

Passed Bend Specimen

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Failed Bend Specimen

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There are three types of guided bend tests:

Root bend tests

Face bend tests

Side bend tests

Bend Test Limitations The same weakness that tensile tests suffer from also affects bend tests. Nonuniform properties along the length of the specimen can cause nonuniform bending. Bend testing is sensitive to the relative strengths of the weld metal, the heat-affected zone, and the base metal.

Many problems can develop in transverse bend tests such as an overmatching weld strength may prevent the weld zone from conforming exactly to the bending die radius, and thus may force the base metal to deform to a smaller radius. This will not produce the desired elongation in the weld. Alternatively, with an under matching weld strength, the specimen may bend in the weld to a radius smaller than the bending die. In this case failure may result when the weld metal ductility is exceeded, and not because the weld metal contained a defect.

These problems with weld strength mismatch can be avoided by using longitudinal bend specimens which have the bend axis perpendicular to the weld axis. In this case all zones of the welded joint (weld, heat affected zone, and base metal ) are strained equally and simultaneously. This test is usually used for the evaluation of joints in dissimilar metals.

Weld discontinuities in longitudinal bend tests that are oriented parallel to the weld axis such as incomplete fusion, inadequate joint penetration, or undercut are only moderately strained and may not cause failure.

o TENSION TESTTension tests are performed for the following reasons:

– Test results are used in selecting materials for engineering applications– Tensile properties are frequently included in the material specifications to ensure quality– often tensile properties are measured during the development of new materials and processes so that different materials and processes can be compared. – Tensile properties are often used to predict the behavior of a material under different forms of loading, other than uniaxial tension. The strength and ductility of metals are generally obtained from a simple uniaxial tension test in which a machined specimen is subjected to an increasing load while simultaneous observations of extension are made. If the loading is continued the specimen will eventually break. A typical stress-strain curve that is produced from a tension test is shown in the diagram.

In a welding application, tension tests involve applying a load to the ends of a standard test specimen and recording the point at which the specimen fails by permanent shape change (yielding) and by fracture. A number of mechanical properties can be determined from a tension test, including the following which are of particular significance in welding:– yield strength ( the stress at which permanent deformation occurs)– ultimate strength (the highest stress the material is able to withstand)– breaking or fracture strength(the stress at which the material fails by breaking)– ductility (the percentage of elongation or reduction of area of a defined segment of the specimen)

Two specific types of tension test specimens are used extensively in testing welding materials and welded joints. One of these uses specimens taken from the weld material only (all weld metal tests), and the other uses specimens taken across the weld(reduced section tension specimens). The latter specimens are machined so that the smallest dimension of width is in the weld area (reduced section tension test).– All Weld Metal TestThis test is used to determine the tensile properties of a specimen that consists entirely of weld metal. The test specimen is oriented parallel to the weld axis, and is machined entirely from the weld metal. There are two reasons for performing an all weld metal test: – to qualify a filler metal or– determine the properties of the weld metal in a particular weldment. To qualify a filler metal the melting of the base metal is minimized when making a test weld. This procedure is described in the various filler metal standards. If the purpose of the test is to determine weld metal properties in a particular weldment, then the welding process and procedure used in the actual fabrication should be employed to make the test weld. The following are typically properties that are measured and reported in an all weld metal tension test.– tensile strength– yield strength– elongation

Transverse weld tension test specimen

– reduction of area

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All weld metal test specimen

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Longitudinal weld tension test

o HARDNESS TESTING

Hardness can be described as the ability of a material to resist permanent or plastic deformation, and is usually measured by its resistance to indentation by an indenter of a standard shape and size.

The hardness test is by far one of the most valuable and the most widely used mechanical test for evaluating the properties of metals as well as certain other materials. In general, an indenter is pressed into the surface of the metal to be tested under a specific load for a definite time interval, and a measurement is made of the size or depth of the indentation.

The main purpose of the hardness test is to determine the suitability of a material, or the particular treatment to which the material has been subjected to.

Hardness testing may be used alone or to complement other test methods. This is what makes the hardness method so popular because of the relationship that exists between hardness and other properties of the material. For instance, both the hardness test and the tension test measure the resistance of a metal to plastic flow. Such correlations are approximate and must be used with caution when applied to welded joints or any metal with a heterogeneous structure.

It should be noted that hardness is not a fundamental property of a material and a hardness value is an arbitrary number. There are no absolute standards of hardness and it has no quantitative value, except in terms of a given load applied in a specified manner for a specified duration and a specified penetrator shape.

Measurements of hardness can provide information about the metallurgical changes caused by welding. For example, in alloy steels a high hardness could indicate the presence of untempered martensite in the weld heat-affected zone, while a low hardness may indicate an over-tempered condition. In cold-worked or age-hardened metal, welding may result in significantly lower heat-affected zone hardness due to recrystallization or over aging.

Hardness testing is divided into two categories: macrohardness and microhardness

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Hardness scan – fillet welds

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Hardness scan – butt weld

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﻿The hardness testing methods in use today for testing metals are: – Brinell– Rockwell– Vickers– Knoop o CHARPY IMPACT TESTThe Charpy vee-notch impact test is the most common fracture toughness test used by industry. A notched specimen is broken by a swinging pendulum and the amount of energy required to break the specimen is recorded in foot-pounds or joules. This is determined by measuring how far the pendulum swings upwards after it fractures the specimen. If the specimen is tough, the pendulum will only swing up a small distance since part of its energy has been absorbed by the specimen. If the specimen is brittle it will absorb little energy thus allowing the pendulum to swing up to almost its original height﻿

Charpy Impact Testing Machine

The amount of energy absorbed can be read directly off of the dial indicator that is located on the machine.The specimen is supported in place as shown and the pendulum strikes it from behind the notch.﻿ ﻿

Charpy vee-notch specimen holder

This puts the notch in tension, causing the specimen to fracture. The dimensions of the specimen are shown in the next diagram. In some cases sub size specimens may be used when the material thickness is to small to accommodate the full size specimens. It is extremely important that the specimen is machined to the tolerances and finishes specified (eg ASTM E23 Standard Methods For Notched Bar Impact Testing Of Metallic Materials).﻿

Charpy vee-notch specimen dimensions

﻿ ﻿Metals such as carbon and low alloy steels, exhibit a change in failure mode with decreasing temperature. For this reason, it is common to conduct impact tests over a range of specimen temperatures. The performance of the material at different temperatures can be observed and a conclusion made regarding the temperature below which the material can no longer be used without a risk of brittle fracture. The graph shows the relationship between test specimen temperature and absorbed energy

Impact energy vs temperature

The absorbed energy is the most common value reported, however, the percent shear and the lateral expansion may also be noted. Metals that exhibit a high Charpy vee notch value are typically those that are more resistant to brittle fracture. It is important to remember that these tests are comparative only and are no guarantee of ductile behaviour in actual service. The fractured ends of a specimen often reveal the manner in which it fractured. If the specimen has fractured in a brittle manner with low energy the faces will have a flat, crystalline and shiny surface. A tough specimen will exhibit more deformation and will have a dull and fibrous surface.

Fractured charpy vee-notch specimen

o IZOD IMPACT TEST

The Izod test is another form of impact testing. It also involves the use of a vee notched specimen and a machine to deliver an impact blow to the specimen. Testing is generally carried out with the specimens at room temperature since the time required to accurately place it in the machine allows its temperature to increase. This can introduce a significant error when conducting tests at various temperatures.

The positioning of the specimen within the testing machine is critical. Unlike the Charpy specimen, the Izod specimen is held rigidly in a vice type fixture with the notched side facing the direction of impact. The centerline of the notch must be in the plane of the vice top within .125 mm. Once the specimen is in place the hammer is released from a preset height and allowed to strike the specimen thus fracturing it at the vee notch